Heating systems employ various methods to control the temperature of components with the system. The temperatures of these components are usually regulated within a particular range in order to maintain safe operation. Two such components that require regulation are heat exchangers of furnaces and the water inside a pressurized hot water boiler. Redundant sensors are often used in safety-related components such as these, which provide greater confidence that the sensors are operating properly. Two or more such sensors may reduce the probability of the heating control system recognizing an incorrect temperature, however, the proper functionality of the additional sensors are not known with any greater confidence than the initial sensor.
Temperature measurement is important in many such processes. A common method of temperature measurement uses thermocouple transducers that output an EMF in response to a temperature gradient across two dissimilar materials, typically metals. It is well known, however, that thermocouples degrade over time due to chemical and metallurgical changes in the composition of the materials. Various thermal sensors and detectors such as thermistors, platinum resistance elements, and other types of temperature sensors are also utilized in many heating, ventilation, and air-conditioning (HVAC) applications.
Most temperature sensors used in these HVAC applications, whether used in industrial, commercial, or residential markets, eventually suffer from some form of serious degradation and/or failure of the sensor. Such degradation or failure modes of temperature detectors include thermal degradation, metal fatigue, corrosion, chemical and mechanical changes, which may render the sensor inoperable or otherwise induce a system failure.
During the use of thermocouples, for example, several forms of degradation take place in the thermocouple circuit including chemical, metallurgical, and mechanical changes in the materials and elements or devices of the circuit. Such changes may be accompanied by a shift in the resistivity of the thermoelement, thereby indicating a false temperature measurement.
Heating applications likely produce the greatest potential for sensor failures, where the sensor is particularly susceptible to extremes of thermal degradation and chemical changes. These sensors may include temperature, pressure, flow, and medium presence sensors, and others such as may be used in furnaces and boilers. The exposed portion of the sensor is often the hottest portion of the measurement circuit and may therefore be exposed to the harshest conditions. The temperature sensor and other related sensors are also exposed to processes that may increase the likelihood of changes in the electrical properties of the sensor or cause a complete system failure.
In boiler applications, for example, temperature, pressure, flow, and medium presence detection may be used, wherein the failure of a temperature sensor or an associated low-water level cutoff detector may cause a boiler malfunction or failure. Thus, the failure of such boiler sensors poses a problem. In furnace applications, the temperature sensors and/or limit detectors used in a heat exchanger of a furnace may also reach very high temperatures, and cause overheating conditions that could cause the system to fail. Accordingly, a fail-safe temperature sensor and/or a fail-safe low-water level cut-off detector would be desirable to avoid such problems.
For design, manufacturing, and applications reasons, the HVAC sensors discussed above are generally individually fabricated, packaged and mounted. However, the use of these numerous individual sensors also requires more system mounting difficulties and added complexity in support of the remaining portion of the control system. Such additional support components and circuitry may include related relays, power supplies, and microprocessors that increase the overall cost and complexity of the system.
In many applications, however, several specific sensors are commonly used together. For example, in the case of boiler heating systems, a boiler water temperature sensor is usually accompanied by a low-water cutoff detector, which senses the presence of the water (or another such medium) when strategically placed at the low water level of the boiler. If the water falls below this level, the system is typically shut-down until more water is added, thereby immersing the sensor again.
Accordingly, for fail-safe temperature readings, cost, mounting and system simplicity reasons, there is a need for a fail-safe sensor of a temperature monitoring system that incorporates both temperature and medium detection functions in a single housing.